Electric wave transmission system



Oct. 20, 1942. H, R, MOQRE 2,299,487

` ELECTRIC WAVE TRANSMISSION SYSTEM Filed May 3, 1941 2 Sheets-Sheet l Oct. 20, 1942- H. R. MOORE ELECTRIC WAVE TRANSMISSION SYSTEM Filed lay 3. 1941 2 Sheets-Sheet 2 w .mi

Patented Oct. 20,

ELECTRIC WAVE TRANSMISSION SYSTEM Hilbert B. Moore, PluckeminfN. J., assigner to` Bell Telephone laboratories,

Incorporated;

New York, N. Y., a corporation of New York l Application May s, 1941, serial No. 391,657

J 4 claims.

This invention relates to electric wave transmission systems, and, more particularly, to a Y power line carrier frequency telephone system.

Carrier frequency telephony over transmission lines primarily intended for transmission and distribution of low frequency electric power is known.

Buch telephonie transmission has been by means` of a carrier wave or waves amplitude modulated in accordance with the telephonie signal or voice frequency currents constituting the modulating current or wave.

An object of this invention is to improve carrler frequency telephony over power transmission lines. e

'I'here has been an increasing extension of rural power lines in recent years. These line's more frequently than not make use of single phase high ment for the' telephone subscriber stations and theY central oilice for the power line telephone system, usually involves further sizable losses, and still other losses are introduced by bridged power line taps and by connections from branch distribution lines. An ordinary amplitude modulated carrier frequency telephone system operating over such a facility involves fairly high transmission levels, sensitive receiving equipment, and,

Ibecause of widely different losses over the various links of a party line arrangement, automatic gain control. Preservation of an adequate signal-tonoise ratio event with relatively high transmission levels appears to require use of carrier waves of a frequency above about 200 kilocycles inasmuch as the circuit noise increases rapidly below that frequency. k

In contrast, a wide band frequency modulation system by reason of its inherent noise advantage, may be operated at considerably lower transmission levels, effecting a saving in equipment and operating costs. Alternatively, or, also, such a system could be operated at lower frequencies where the line attenuation would be less. Autosuch a system. Transmission distortion because of irregularities in the attenuation-frequency characteristic of the power line, important in the case of an amplitude modulation system, is of lesser concern in the case of a frequency modulated system. y

,- A feature of the invention comprises transmitting and receiving over a power line on afhigh frequency or carrier wave angularly modulated in accordance with telephonie, signaler voice frequency waves.

Another feature of the invention comprises a system and means for enabling carrier frequency telephony over a power line in whichthe carrier wave may either be frequency, phase or otherwise angularly modulated in accordance with the modulation current or wave. l

A power line carrier frequency telephone system in accordance with the invention comprises a transmission line for transmitting electric "power, for example, of high or low voltage and of relatively low frequency, between two or more points and to a plurality of power consumers some or all of whom, together with others in the locality through which the line extends,` desire telephone service. Telephone subscribers stations are coupled to the power line, and are adapted to transmit telephonie signals onhigh frequency or carrier waves.` At each station. and at the central oice for the system,'means are provided for angularly modulating the carrier waves for transmission outgoing from the station or oflice, and for demodulating an angularly modulated carrier wave incoming to the station or office. In the particular system to be described in detail hereinafter, transmission from each sta* tion is on a common high frequency or carrier wave, and receiving at each station 'is on a second carrier wave is located at the central ofiice and is continuously supplying the second carrier wave to the power line. The carrierfrequency equipment at each lstation and the central oilce may be arranged for operation either on a frequency modulation or a phase modulation basis or both.

A more complete understanding of this invention will be deriyed-from the detailed description that follows. read with reference to the appended drawings, wherein:

matic volume control would also be inherent in F18- 1 Shows a port of a power line carrier frequency telephone system, with one telephone station shown in detail;

Figs. 1A and 1B show networks for insertion, under certain circumstances, at points A and B of Fig. 1 and of Fig. 2; and

Fig. 2 shows the circuit arrangement for the central omce of the system of Fig. i.

Fig'. I shows a portion of a power line carrier frequency telephone system comprising a power line I0, a plurality of telephone subscriber stations A, one of which is shown in detail, andl a central oflice CO for the system. Each station may communicate with the other stations in the system by way of the central omce but without the interposition of the operator at the central oflice, and, also, with the operator when desired. The line is shown as a single phase power line with one conductor grounded at a multiplicity of points along its length; although it could be an insulated single phase line, or comprise a pair of phase wires of a multiphase power line, or-

other type of power line.

Each telephone station A comprises a transmitting circuit Il and a receiving circuit l2 constituting branches of the line I3 coupling the station to the power line.

The transmitting circuit comprises a transmitter 20, for example, of the granular carbon type, a frequency control circuit 2|, an oscillator 22, and an output amplier 23. The transmitter is lconnected in series with a source 24 of talking current, one winding of a repeating coil or transformer 25. and the telephone receiver switchhok contact set 26. When the station is not in use, the telephone receiver is supported on the switchhook 21 and the contact set 26 would be open. A low frequency (for example, 20 cycles per second) interrupter 29 is connected in series with the normally open contact set 30 of a signal key, the source 24 andthe primary winding of transformer 25.

The frequency control circuit comprises the multigrid electron discharge device VI having an indirectly heated cathode, an input control grid. a suppressor grid connected with the cathode, a screen grid and an anode. Anode potential and cathode-anode space current is supplied from source 35, bias for the input grid being derived from'the cathode resistor 36, by-passed for alternating current by the condenser 31. A

The oscillator may comprise an electron discharge device V2 having an indirectly heated cathode, an input grid and an anode. The oscillatory network comprises a parallel-connectedA inductance 4I and capacitance 42 between the grid and anode of device V2, the anode of the oscillator being supplied with potential from source 35. The condenser 43 isolates the oscillators grid from the Isource of anode potential. Bias for the grid of device V2 is provided through resistance 44 by the cathode resistors 45, 46, and normally (that is, during a non-talking or nonsignaling period) is of a value suicient to prevent oscillation of the device. A normally open key 41 is connected across resistor 45; when closed, it short-circuits the resistor 45 whereby the oscillators grid bias is reduced suiciently to permit the generation of sustained oscillation of high frequency, say, Fl. Condenser 48 by-passes this high frequency way around the bias resistors.

The input grid of the device VI is-coupled to lthe oscillatory circuit through an anode-potentialy blocking condenser 50 and a resistance 5I, the latter having a value sizably greater than the reactanceof the grid-cathode capacitance of the agence? tube Vi. As a result, the grid excitation of the multigrid device lags the voltage across the network of inductance 4l and capacitance 42 by approximately 90 degrees and so, also, does the cathode-anode current of the device. This lagging current, owing through\inductance 4|, changes the elfective reactance of the oscillatory network, and, consequently, the frequency of the oscillation generated by the oscillator, the change or shift in frequency depending upon the change in the cathode-anode current of the tube Vl. The normal space current for the latter may be varied by choice of the grid biasing resistor 36, and, hence, within limits, the steady state frequency of the oscillator. Speech or signal currents impressed between the grid and cathode o'f the device Vi through the secondary winding of transformer 25 and resistance 52, cause alteration of the grid potential and, consequently, of the cathode-anode current, effecting corresponding changes in the frequency of oscillation of the device V2 to values equally spaced above and below the steady state frequency FI in accordance with the input signal to the tube Vl. The` resultant frequency modulated output is applied by way of the coupling condenser 53 to the 'amplifler 23, which may be arranged for frequency multiplication, if'deslred, and through the line i3 to the power line.

The receiving circuit l2 of a station A comprises a high-frequency tuned amplifler 60, an amplitude limiter circuit 6|, a discriminatordetector circuit 62, an audio frequency amplifier 63, a station signal or ringer 64, an attenuation network or resistance pad 65, and a telephone receiver 66. This circuit is normally in condition for reception, i. e., the components thereof are continually energized.

The tuned amplifier may be'of any conventional type known in the radio frequency art, and is designed to transmit an incoming angularly modulated high frequency or carrier wave F2 and its sidebands, but to reject the high fre- -quency FI and its sidebands, thus making the 'substation circuit anti-sidetone and precluding singing around the substation circuit. The second high frequency F2 will be suiiiciently sep'arated from the high frequency FI used for transmitting from the station so that there is no overlapping of the sidebands of the two carrie waves when angularly modulated.

The limiter circuit comprises an electron discharge device or tube V3`of the multigrid type deriving its anode and its screen grid potential from a source 10, and its input grid biasing potential from the cathode resistor 1I, by-passed for alternating current by condenser 12. The anode or screen potential and the input grid bias for the device V3 are such that the tube overloads for amplitudes of the incoming wave above a preassigned normal level, thereby precluding further transmission of noise or other extraneous components of greateramplitude thanthe carrier wave. The screen grid potential is reduced to desired value by resistance 68, which is by-passed by condenser 69.

The discriminator-detector circuit is of a known type employed in commercial frequency modulation receivers. It comprises a .double `diode V4 the anodes of which are connected through a transformer to the anode-cathode circuit of the tube V3. Each winding of the transformer 88 has a condenser 8|, 82 connected thereacrossftuning the transformer to the high frequency F2. In addition to the inductive coufrequency Fl.

secondary voltage ot transformer 8l, the primary voltage being in quadrature with the secondary voltage at lthe resonant frequency. At frequencies above the resonant frequency, the phase is shifted in one direction and at frequencies below resonance in the other direction, causing the resultant voltages applied to the diode anodes to become unequal, first one and then the other anode being at the higher potential.

The output circuit of the double diode comprises the equal load resistances 95, 95, eachV by-passed for the carrier wave by condenser-s 91, 99, the common terminal of the resistances 95, 96 being connected through the resistance 99, of high impedance to the carrier wave, to the mid-point of the secondary or diode winding of the transformer 89. The rectified voltage developed across the resistances is normally equal and opposite, i. e., when the unmodulated carrier wave F2 is incoming to the diode. When the frequency modulated carrier wave i`s impressed on the double diode, the voltages developed across the resistances 95, 86 will be different,

the resultant varyingvoltage across the resistances corresponding to the variationsv of the modulation component, and, hence, of the original modulating wave. c

The amplifier 63 amplifies the detected modulation component and, dependent on whether the modulation component is a station signaling wave or a speech currents wave, delivers the amplified wave to the station signal 64, or the station telephone receiver 66, through the attenuator 55.

As already noted, each station A is coupled to the power line through a line I3, one conductor of the latter being connected to the grounded wire of the po-wer line, and the other being connected through a condenser 91|v to the non-grounded wire of the power line. This condenser is suitably proportioned so that itoffers a high impedance to the low frequency electric power but low impedance to the carrier waves. A suitable -protector block |09 is connected between the conductors of the line I3. i

Before describing the operation of the station in originating and receiving a call to and from alg-a2'. The escalator zz' is normany conunuously lnoscillation, supplying the high frequency F2 to the line at all times. The output ter-iVI minals of the audio frequency amplifier in the receiving circuit l2 are connected to a low frequency (for example, cycles per second) relay 9|, and through an attenuator or resistance pad 92 to the 'input terminals of anaudio frequency another party on the power line system or to and from the operator at the central ofce, the arrangement at the central office will be described.

Each subscriber station is arranged for transmitting on one high frequency wave FI and for receiving on a second high frequency wave F2. The central oilice, as shown in Fig. 2, is arranged however, for transmitting on the seco-nd high frequency wave F2, and'for receiving on the first high frequency wave FI. So far as the carrier wave equipment is concerned, the central office and the subscriber station A are the same, except that the steady state frequency developed byl the oscillator 22 in the transmitting circuit Il' is that of the carrier Wave F2, and in that the high frequency tuned amplifier 90 is` tuned to a band centered on the high and capacitance 42' are of appropriately different values, as are those of the coupling transformer 80 and its associated tuning condensers amplifier 93, for example, of the voice operated gain adjusting type, and to an operators line jack 94. The high frequency .portion ofthe central office arrangement need not be housed within the office, but maybe located at any convenient point, for example, in a housing adjacent to the power line used as the ltransmission path. In such a case, connection to the ofllce would be over aI voice frequencyline interposed between the operators jack 94 and the remainder of the equipment. Signaling could be relayed from the relay 9| to the signal lamp 95 over a simplex circuitV consisting of the voice frequency line with ground return.

Let it be assumed that a subscriber vat one station on the power line wishes to establish a connection with lanother subscriber at a second station onthe power line. The circuit arrangement at the calling station would be that shown in Fig. l. The calling party operates the key 41 4to close its contact set. thereby short-circuiting the interrupter circuit in accordance with the calling code for the system. Each time the signal key is closed, the low frequency generated in the primary winding of the transformer 25 by the operat.on vof the interrupter induces the signal wave in the secondary winding and across the grid-cathode circuit of the tube VI. The resultant changes in the anode current of the device VI change the effective reactance of the oscillatory circuit of the oscillator, and vary the oscillator frequency in accordance with the signal wave. The high frequency wave FI, together with its sidebands, is amplified in amplifier 23 and transmitted to the power line I9, and over it to the central office. Here the frequencyv modulated carrier wave is transmitted by the tuned high frequency amplifier 60' to the amplitude limiter circuit 6| and thence to the discriminator-detector circuit 62' in which the signal wave modulation component is detected and transmitted to the audio frequency amplifier 63'. The output of the amplifier operates the relay 9| causing the signal lamp 95 to operate. Simultaneously the amplifier output is applied to the amplifier 93. The amplified audio frequency output is impressed on the frequency `control circuit 2|', and by causing variation in the eective reactance of the oscillatory circuit of oscillator 22' frequency modulates the high frequency wave F2 generated by the'oscillator 22', The high frequency wave F2 and its sidebands are amplifled in the amplifier 23', and transmitted to the power lineand over the latter to each of the otherystations A coupled to the power line. Since the receiving circuit at each station is continuously energized, i. e., in condition to receive, the signal wave modulated high frequency wave F2 is transmitted by the rtuned amplifier 69, limited in amplitude in the amplitude limiter circuit 6| and demodulated in the discriminatordetector circuit 62. The reconverted audio frequency signal wave is amplified in the amplifier 43, and transmitted to the signal device 64 to operate it. The device 64 operates each time and during the time the signal key at the calling station is closed. When the calling party ceases to signal, the party at the station whose code has been signaled, responds by removingV his receiver from its hook, closing his normally open key 41, and beginning to talk. During conversation, each party transmits on the carrier wave FI and receives on the carrier wave F2, the frequency modulated carrier wave transmitted from each station passing through the central ofiice where it is demodulate'd and the demodulation component remodulated on the second high frequency wave that each station isadapted to receive. In each case, the carrier wave will be frequently modulated in accordance with the voice frequency currents in the transmitter circuit at each station when the party thereat talks into his transmitter and generates such voice currents corresponding to the sound waves acting on the transmitter diaphragm. When the conversation is completed, each party restores his station to its normal or receiving condition by restoring the key 41 to its normally open condition by replacing his receiver 66 on its switchhook 21.

If the calling party desires to establish connection with the operator at the central oilice instead of with another subscriber on the power line, he operates his signal key (after closing the key 41) in accordance with the code for the central oflce. The operator responds by inserting the plug of the operators set (not shown) in the line jack 94 and beginning the conversation. The

, operator transmits on the high frequency wave F2, and receives on the high frequency wave Fl. The central-office may be equipped with conventional facilities for connectingthe subscriber with a general telephone system. When a call is incoming to the central oiiice from the latter system, the operator signals the desired power system telephone station by operating her low frequency ringing key in accordance with the code for the power line system, the signaling taking place on the high frequency wave F2, frequency modulated in accordance with the low frequency signal wave.

i Although the system described specifically hereinabove involves the use of a high frequency or carrier wave frequency modulated in accordance with the signal or the voice currents wave, the advantages of such a system may also be achieved by use of other methods of angular modulation, more particularly, phase modulation. The fundamental difference between a frequency and a phase modulated wave is that in the frequency modulated wave the frequency shift or deviation produced by the modulation is independent of the modulating frequency while in the phase modulated wave, the frequency deviation is directly `proportionalto the modulating frequency. It is apparent that the introduction of a suitable network in the audio input to the frequency control circuit of a frequency modulated transmitter will result in the production of a phase modulated wave. Such a network, 'indicated in Fig. 1A, having an output whose magnitude varies in direct proportion to the frequency of the input thereto, would be inserted at Vthe points A in the station and central office transmitting circuits. Demodulation of the phase modulated wave may be accomplished with a circuit like that used for demodulation of frefrequency output of such a demodulation circuit varies in direct proportion to the demodulated frequency, the network indicated in Fig. 1B should be inserted at points B in the station and central onice receiving circuits. This network should have an output that varies in inverse proportion to the frequency of the input thereto.

In particular situations it may be desirable to utilize a modulation that is neither phase nor frequency modulation per se. In such an event, a network might be inserted at points A having an output that is independent, of frequency at the lower modulating frequencies, but that increases in proportion to frequencyat the higher modulating frequencies. A-restoring network of complementary characteristics would be inserted at points B. More generally, any pair of predistorting and restoring networks having complementary output versus frequency characteristics might be employed, whether to achieve lmproved signal-to-noise ratios, -or for any other purpose.

What is claimed is:

1. The combination of a power line for transmitting electric power, a plurality of telephone stations coupled to said power line, each of said stations adapted to transmit on a first high frequency electric wave and to receive on a second high frequency electric wave, means at each station to generate said first high frequency electric wave and to frequency modulate the electric wave in accordance with voice frequency currents corresponding to sound waves, means at each station tuned to a frequency band centered on said second high frequency electric wave and adapted to demodulate an incoming frequency modulated electric wave of said second frequency, a relay telephone station coupled to said power line, means at said relay station tuned to a frequency band centered on said first high frequency electric wave and adapted to demodulate a frequency modulated electric wave of said first frequency, and means at said relay station to generate said second high frequency electric wave and to frequency-modulate it in accordance with the demodulation components of the frequency modulated waves. but, because the audio 76 quency modulated lelectric wave incoming to the relay station.

2. The combination as claimed in claim l in which the high frequency electric wave generated at said relay station is supplied-continuously to the transmission line.

3. The combination of a line for transmitting electric power and means coupled to said line for telephonie communication over said line between separated points thereon, said means including a telephone station, said station comprising a transmitting circuit including an audio frequency transmitter, and a receiving circuit including an audio frequency telephone receiver; said transmitting circuit including an oscillator for generating high frequency electric waves, means under the control of the user of said station` to set said oscillator to generating a single high frequency electric wave and to transmit it to said power line, means coupled to said audio frequency transmitter for modifying the output of said transmitter when audio frequency currents are generated in the latter and means responsive to said modified transmitter output for varying the frequency of said single high frequency electric wave; and said receiving circuit comprising a high frequency amplifier tuned to va frequency centered on a second high frequency electric wave, means for limiting the amplitude of such a wave received in the receiving circuitfrom said power line, a detector circuit for translating the modulation component of such a band of high frequency waves into audio frequency electric waves for delivery to said telephone receiver, and an audio frequency electric wave distorting means in circuit between said detector circuit and the telephone receiver.

4. The combination of a line for transmitting electric power and means coupled to said line for telephonie communication over said line between separated points thereon, said means including a telephone station, said station comprising a transmitting circuit including an audio frequency transmitter, and a receiving circuit including an audio frequency telephone receiver; said transmitting circuit including an oscillator for generating high frequency electric waves, means under the control of the user of said station to set said oscillator to generating a single high frequency electric wave and to transmit it to said power line, a network coupled to said audio frequency transmitter and having a characteristic such that the magnitude of'its output varies in direct proportion to the frequency of the input to the network from said transmitter, and means responsive to the output of said network for varying the frequency of said single high frequency electric wave; and said receiving circuit comprising a high frequency amplifier tuned to a frequency centered on a second high frequency electric wave, means for limiting the amplitude of such a wave received in the receiving circuit from said power line, a detector circuit for translating the modulation component of such a band of high frequency waves into audio frequency electric waves for delivery to said telephone receiver, and a network connected in circuit between. said detector circuit and the telephone receiver, said network having a characteristic such that the output from said network varies in inverse proportion to the frequency of the input thereto. A

HILBERT R. MOORE. 

